Network analyzers are devices that are used to determine the radio frequency (RF) characteristics of various devices under test (DUTs). In many situations, a DUT is a relatively small component designed to interface with a trace contact point on a printed circuit board (PCB). Many network analyzers typically utilize an interface adapted to receive a coaxial coupling. To test a DUT designed to be employed on a PCB using such a network analyzer, a test fixture is often employed. A test fixture is generally a specialized device that is adapted to readily accept a DUT and that electrically couples the DUT to one or several ports of a network analyzer.
For many DUTs (such as balanced filters, baluns, balanced amplifiers, etc.), the pertinent performance measurements depend upon both the magnitude and phase of the signals applied to and received at each port. In the case of balanced devices, it is quite important that the phase of delay of each test fixture port be identical between the balanced pairs of ports. However, the use of network analyzers and test fixtures to perform such measurements presents difficulties. Specifically, it is common to experience different path lengths on different ports using test fixture/network analyzer configurations. The variations may result from PCB layout constraints, manufacturing process limitations, and/or other reasons.
“Port Extensions” for network analyzers have been developed that attempt to address the difference in path length between ports of a network analyzer. A port extension provides a mathematical delay to the results measured on a respective port. The mathematical delay models the linear portion of the phase response that results from the differences in electric length of a test step-up.
Specifically, port extension functionality typically operates by defining a respective delay value for each port. After measurements are made, the network analyzer applies phase compensation to the measurement data using the defined delay values depending upon the ports involved. For example, when reflection measurements are made, twice the delay defined for a given port is used to compensate for the delay in the incident signal and the reflected signal. For transmission measurements, the delay of the incident port and the delay of the response port are used to compensate for the delay of the electrical path between the two different ports. Each phase compensation value for the various frequencies of the frequency span is then calculated by multiplying the total delay by the respective frequency of the phase compensation value. Accordingly, by suitably applying the phase compensation values, the data provided by the network analyzer compensates for the delay introduced by the test fixture.